You know that an adventure series has really hit its stride when you find yourself enjoying the expository sections — those quiet moments when the characters are just talking — just as much as the rock-’em, sock-’em action sequences. I’m thinking of books like “Harry Potter and the Goblet of Fire,” when the Potter series graduated from cute magical whodunits to a story about characters whom you suddenly realized you knew intimately and cared about deeply.

Right on schedule, it seems Matthew Phillion’s Indestructibles books have cleared that hurdle: With Book 3, “The Entropy of Everything” (2015) and Book 4, “Like A Comet” (2016), the teenage heroes of the YA series’ first two installments have moved well beyond the comic book tropes that bore them and blossomed into full-blown, flesh-and-blood people, with all the messy complications that entails.

In “Entropy,” the gang has to travel into the future to save an alternate timeline where the Indestructibles — one of them in particular — play an unfortunate role in the end of humanity. That provides some rollicking good fun, full of killer robots and dystopic hellscapes. (“This place looks like the movie sets of Star Trek and Rent mated and had a baby,” notes gravity guru and pop culture maven Entropy Emily.)

But it also offers a chance for the characters to grow and change as they come face-to-face with the challenges and mistakes (and in some cases, the bitter ends) of their alternate-future selves.

“Entropy” also takes a major leap forward in developing Titus, the group’s teenage werewolf, and his extended lupine “family,” exploring their outsider status and their legacy of violence — not to mention Titus’ complicated but tender relationship with Kate, a.k.a Dancer, the group’s stoic street vigilante. The werewolves really go to town in this book, engaging their future foes with a savagery that’s new to the series but feels wholly earned.

Titus and Kate’s relationship is also central to “Comet,” where the teens take on an invading alien force and several other Indestructibles also get a chance to shine: Straylight, the teen/alien symbiote combo, achieves full heroic potential on a daring space mission, and Emily gets to fulfill a lifelong dream I won’t give away, except to say that if you’re reading a series of superhero novels, it’s probably your lifelong dream too.

In many ways “Comet” feels like a culmination, as pretty much all the characters we’ve met over the first three books converge to face the world-ending alien threat. That includes the team’s magical mentor Doc Silence and his crew of older heroes (including Korthos of Aramais, a sort of cross between Conan, Thor and Drax the Destroyer who’s just crying out for an origin story), along with various anti-heroes and even some villains who band together to try to save the planet.

It’s a welcome theme of unity at this troubled time in our nation’s history, and Phillion does an amazing job of juggling all the characters and their plotlines.

It’s that deft plotting and precise character work, combined with knowing winks at his pop culture predecessors and action sequences that continue to dazzle in their detail and scope, that make these YA novels accessible to anybody, from the starry-eyed 11-year-old to the middle-aged former comic book reader who never quite grew out of it. (We know who we are.) Four books in, Phillion has guaranteed that wherever this series goes, he’ll have a following of loyal readers who will go with it.